The present invention relates to high strength heat resistant low alloy steels adapted to use for the material for, such as, power plant boilers, heat exchangers and pipes in chemical plants, forged and cast steel products, such as, high temperature pressure valves etc., various steel half-products, such as, round steels, profiles, slabs and plates for products of manufacture for high temperature uses, such as, hooks, suspensions, tensile members, support members, etc.
Heretofore, various heat-resistant steels have been in practical uses, including austenitic stainless steels, 9% chromioum steels, 12% chromium steels, 1-21/4% chromium steels and low chromium steels of less than 1% chromium.
In these conventional heat-resistant steels, problems are left unsolved in using them at high temperatures up to about 600.degree. C., such as follows:
1 Austenitic steels:
While these steels exhibit in general better performances as to the high temperature strength, toughness and workability, they tend to suffer from stress corrosion cracking and grain boundary corrosion in certain application conditions. Their higher prices may be accounted as an additional disadvantage.
2 9% and 12% chromium steels:
Among them, STBA 26 (9% chromium 1% molybdenum steel) and X20CrMoV 121 (12% chromium 1% molybdenum vanadium steel) of DIN standard have a higher carbon content of about 0.13-0.25% by weight and, hence, are apt to suffer from occurrence of weld crack and exhibit poor workability. In the recently developed low carbon steels having contents of V and Nb, weldability and high temperature strength are improved as compared with the high carbon steels mentioned above. They exhibit, however, lower heat conductance and are, in general, poor in workability upon welding.
3 1%-21/4% chromium steels:
These steels have better resistance against oxidation, permitting their use at temperatures up to about 600.degree. C. They are most excellent in high-temperature strength over the low alloy steels inclusive of STBA 26 and are better also in weldability and workability. However, the high-temperature strength of these alloys does not surpass those recently developed high strength steels of 9% chromium steels, of 12% chromium steels and of austenitic stainless steels. Thus, it is necessary to design various parts to be employed at about 600.degree. C. with considerable thicknesses, so that it is unavoidable to endure occurrence of large thermal stress in large diameter pipes in, for example, high-temperature pipe lines etc.
4 Low alloy steels having chromium content less than 1.0%:
They reveal lower high-temperature strength and are poor in resistance against oxidation as compared with those of 1%-21/4% chromium steels, so that they possess lower maximum operable temperatures.
In steels, in which a small amount of V or Nb is contained in order to improve the high-temperature strength, local portions having finely dispersed microstructure due to recrystallization caused by a possible welding heat or so on exhibit decreased hardness as compared with the original material. When testing a test specimen having such a local low hardness portion on a creep rupture testing machine or on a tension tester, it will be broken at such a portion and shows a lower strength value than the original material.
Low carbon steels of 1%-21/4% Cr having contents of Mo, W, V and Nb have a large proportion of ferritic phase and exhibit lower toughness.